m, Peter

Abstract [en]

Porous oxide-oxide composites are interesting high temperature materials. Their natural oxidation resistance and good toughness makes them suitable for various high temperature applications. One application is aeroplane engines where higher temperatures would enhance efficiency and in turn decrease fuel consumption and emission. The high temperature properties of the first commercial porous oxide-oxide composite Nextel 720/aluminosilicate has been evaluated. Tensile testing showed a significant embrittlement after heat treatment at 1100¢ªC or 1000¢ ªC and long exposure times. The main object of this report was to determine the reason for this embrittlement. The microstructure was studied thoroughly to see any differences between as- received and heat treated samples. These investigations showed a decrease in nano-porosity in the matrix and also an increase in macro-porosity. High temperatures are a driving force for densification. The fibre network constrains the matrix and prevents it from shrinking. The driving force for sintering and the constraint by the fibres causes the nano-porosity to decrease and macro-porosity to increase. The nano-porosity governs the toughness of the composite and the decrease in nano-porosity is the cause of embrittlement.